1 - Field of the invention
The present invention relates to picture transmission in general, and particularly to animated pictures such as television pictures. More specifically, the invention is concerned with a method of decreased rate picture coding and increased rate picture decoding, so as to transmit a digital signal of animated pictures through a variable :ate digital network such as an asynchronous digital network.
2 - Description of the Prior Art
In the article entitled "L'image numerique et le codage" (Coding and the digital picture) published, i.e. parenthesis in "L'echo des recherches", No. 126, fourth quarter 1986, pages 21 to 36, J. GUICHARD and D. NASSE describe the state of the art in the field of digital picture processing for decreased rate picture transmission. The different coding and decoding devices in existence are designed for picture transmission through constant rate transmission paths, such as those going through an integrated services digital network (ISDN). The coding device is usually equipped with a buffer memory of adequate capacitance at output so as to match the coded picture rate with the constant rate of the transmission paths allocated to the network. This buffer memory introduces a dead time in picture transmission between the transmission equipment and reception equipment situated at different extremities of the network and including respectively a coding device and a decoding device. This dead time in picture transmission is particularly inconvenient, e.g. in the case of interactive picture services such as very low rate telephony (64 kbit/s). The advantage of a variable rate network is that in providing the possibility of transmitting a variable information rate, it enables the capacitance of the buffer memory to be reduced or ever suppressed, thus reducing dead time in transmission between the coding device and the decoding device. Nevertheless, the traffic dispatch capacity of variable rate networks, such as asynchronous networks, is statistically calculated. The fact that all subscribers do not simultaneously use the maximum rate allocated to them is taken into account, and the network is designed to dispatch a statistically determined average total rate. As a result of this, a subscriber can find himself temporarily denied an increase in rate in the event of the network being overloaded and the requested rate being greater than a minimum guaranteed rate. The additional information resulting from the requested increase in rate is not routed to its final destination in this case. In the case of coded picture transmission, the non-routing of the requested additional information is particularly catastrophic since the coding and decoding algorithms are of the recursive type. It is necessary that information that has been processed by the coding device arrive at destination in the decoding device.